Applicants claim priority under 35 U.S.C. xc2xa7119 of German Application No. 100 22 597.7 filed May 10, 2000.
1. Field of the Invention
The invention relates to a device for detecting the marginal edge and marking of a moving web of material.
2. The Prior Art
A device for detecting markings on a moving web of material is known from German Patent Application No. 195 19 607 A1. This device detects lines imprinted on webs of material. These lines correctly guide the web of material and align the treating devices such as the cutting devices. This is important when the edges of the material web do not extend in a clean manner, or have an imprinted image that is not properly aligned with the edges of the web.
This known device comprises a CCD camera that optically scans a defined area of the material web. A light source is associated with the CCD camera, and the light is aimed at the area of the material web that is being scanned by the camera. The light source and the camera are aligned in relation to one another such that the camera exclusively receives light that is diffusely reflected by the material web to avoid glare effects. However, this known device has the disadvantage that material webs having a mirror-like reflection (webs provided with dark markings) do not supply an adequate contrast to the camera, thereby making scanning markings very difficult.
An object of the present invention is to provide a detecting device that can be used to detect the marginal edge or markings on a moving material web.
These and other objects are accomplished by providing a detecting device comprising at least one optical sensor for scanning the material web transversely in relation to its moving direction. A CCD camera is provided as the sensor for generating a two-dimensional image of a defined area of the material web. However, due to the constant movement of the material web in the running direction, it is sufficient to use a CCD line camera that only scans a pixel line extending transversely to the moving direction of the web. The given position of the marking extending longitudinally in relation to the material web can be detected at any time. The CCD-line camera offers the advantage that the information it supplies can be evaluated in a simpler manner and thus more rapidly. This allows the individual measuring cycles to follow each other more closely in time.
At least two light sources are associated with the optical sensor for illuminating the area of the material web being scanned by the optical sensor. One of the two light sources generates a substantially directed light and the other generates a diffused light. The two light sources can be used alternately or at the same time to safely scan markings on webs of different materials. If the material web contains a very rough surface, the impacting light is substantially reflected in a diffused manner, so that the light source generating a directed light is preferred.
Because of its directional effect, the light source generates a narrowly defined speck of light on the material web that can be easily coordinated with the detection range of the optical sensor. This allows the light emitted by the light source to be optimally exploited. To keep interfering reflections of the material web away from the optical sensor, the light source is aligned with respect to the material web and to the optical sensor such that the condition of reflection is not satisfied, i.e. the light beam reflected by the material in a mirror-like manner will bypass the optical sensor. On the other hand, for scanning material webs having a mirror-reflection, a diffused light source is used because the light is emitted from the area where it exits from the light source in all different directions. The condition of reflection of which an angle between the light source and the material web has to be equal to the angle between the optical sensor and the material web is always satisfied for any one of the diffusely emitted beams of light. This allows the surface of the material web having a mirror-like reflection to appear bright in the optical sensor. On the other hand, markings applied to the surface that have a mirror-like reflection (markings that are dark or are reflected in a diffused manner) appear dark so that an adequate contrast is available for detecting such markings. The use of diffused light results in the advantage that the light source and the optical sensor can be aligned with respect to the material web within a broad angular range. Therefore, corrective adjustments of the device with respect to the material web can be dispensed with. In material webs that are very difficult to detect, such as textiles with interspersed metal threads, an applied marking may not supply an adequate contrast with directed or diffused light. In such cases, the applied marking can still be detected with adequate contrast by simultaneously irradiating the material web with both directed and diffused light and the angular position between the device and the material web is not critical.
For simple generation of diffused light, it is advantageous if the light source contains a diffuser disk. The diffuser disk is fully illuminated so that the light is scattered from each point of the diffuser disk in three-dimensional directions. Therefore, the scattered light is optimally diffused and is used for scanning material webs having a mirror-like reflection.
In the textile and paper industries, it is necessary to reliably protect the light sources and the optical sensor against dust and similar elements. This is accomplished by providing a transparent cover that covers both the sensor and the light sources. To form the diffuser disk, the part area of the cover emitting diffused light is provided with a rough surface. The cover is preferably made of glass or a transparent plastic and the rough surface is produced by etching.
To achieve uniform illumination of the diffuser disk, the light source emitting diffused light contains a plurality of emitters. The number of emitters is dependent upon the size of the diffuser disk. The individual emitters generate light cones that overlap one another to further enhance the homogeneity of the diffused light.
The light cones of the emitters have opening angles of at least 45xc2x0 to provide an adequate homogeneous illumination of the diffuser disk and consequently a good quality of the diffused light is generated. The light source can be structured in a compact manner.
To correctly coordinate the light emitted by the first light source that generates directed light, it is advantageous if the light source generates light cones having an opening angle of no more than 60xc2x0. The opening angle is preferably 45xc2x0, which results in a floodlight-like effect of the light source.
If light-emitting diodes are used as the light sources, the light source has a particularly long useful life and requires low energy consumption. Furthermore, light-emitting diodes have a very small light-emitting area so that unidirectional light can be generated with such LEDs. Moreover, light-emitting diodes can be rapidly switched on and off so that the change-over between the different light sources can be made very quickly.
To safely detect material webs of all colors and with all sorts of markings, it is favorable if the light sources emit multicolored light. Therefore, it is possible to even detect markings that are distinguished from the material web not only on account of their brightness, but exclusively by their color. If light-emitting diodes are employed as light sources, it is possible to use multicolor light-emitting diodes. Furthermore, light-emitting diodes with different colors next to each other can be used, and they are combined to form a common light source. However, it is preferred to use white light light-emitting diodes. These LEDs comprise a light-emitting diode emitting blue light and contain fluorescent substances in their housing. These fluorescent substances convert a portion of the blue light into light having a greater wavelength. This white light is produced in the superposition of the total light emitted. The use of white light offers the advantage that each marking that can be recognized by the naked eye is detected by the optical sensor as well.
For the purpose of detecting markings on very difficult material webs, such as webs of knitted materials interspersed with metal, it is advantageous if the light sources can be adjusted with respect to their brightness and mixed with one another. Therefore, it is possible to continually adjust the relative component of the diffused light in the total light so that a high contrast between the marking and the material web is produced.
To find the optimal mixing ratio between the light source emitting diffused light and the light source emitting directed light, a controlling device is connected to the light sources. This controlling device optimizes the contrast between the material web and the marking so that a person is not needed for such purpose. The controlling device is preferably a program-controlled microcomputer or microcontroller.
It may suffice to reverse between the two light sources and determine with the help of the signals supplied by the optical sensor in which case the contrast of the marking is stronger. The light source supplying the higher contrast is then selected while the system is in operation. However, if the two light sources are mixed with each other it is favorable if the mixing ratio of the two light intensities is determined by a successive approximation influenced by the contrast. Particularly when imprinted material webs are used, it may be difficult to separate the marking that has to be used for controlling or adjusting the run of the web from other imprints on the web. In this case, the device supplies a number of possible positions of the marking for which the correct one has to be selected by hand. To facilitate such a selection, it is favorable that the device is equipped with a light indicator projecting onto that position of the material web that corresponds with the marking found. The operator can easily check whether the position found corresponds with the relevant marking, and can instruct the device, for example by depressing keys, to control another found position. Therefore, safe follow-up of the marking is assured even under more difficult conditions.